Cellular phones (handsets) are increasing in complexity as handsets incorporate more types of service. In the United States, it is typical for a handset to work with both Code Division Multiple Access (CDMA) type modulation as well as the older Frequency Modulation (fm) type signals. These different modulation schemes both coexist in the same signal band and exist in different bands. The handset must be able to transition from one to the other. New handsets will be required to work with both CDMA and GSM type modulation schemes, and be able to transition from one to the other.
There are four major GSM standards. The low band contains two standards: GSM 850 and GSM 900. The high band also contains two standards: GSM 1800 and GSM 1900. GSM 850 operates in the United States, in the cellular band. Transmission (Tx) is 824 to 849 MHz, reception (Rx) is 869 to 894 MHz. GSM 900 operates in Europe, with Tx from 890 to 915 MHz, and Rx from 935 to 960 MHz. GSM 1800, also known as DCS, also operates in Europe. Tx is from 1710 to 1785 MHz, Rx from 1805 to 1880 MHz. GSM 1900 operates in the United States in the PCS band, with Tx from 1850 to 1910 MHz, Rx from 1930 to 1990 MHz. The modulation scheme WCDMA operates in most of the world (with the US the major exception), in the UMTS band. Tx is from 1920 to 1980 MHz, Rx from 2110 to 2170 MHz.
Groupe Speciale Mobile (GSM) type phones are becoming the defacto global standard. Consequently, handsets that enable the GSM scheme must also support other schemes. To illustrate, 3rd Generation (3G) handsets must support both GSM and WCDMA. Both modes operate simultaneously, communicating in one mode and searching for service in another. Prior art architecture requires complex timing to avoid self interference.
As shown in FIG. 1, current 3G front-end circuits consist of two main components: the switch and the filters. The switch toggles between transmission (Tx) and reception (Rx) in either GSM mode or to the duplexer for the WCDMA in the UMTS band
After the antenna receives the signal, the signal path is split into two by the diplexer. In the simplest form, the diplexer is a pair of filters: one low pass to select for the low GSM bands, and one high pass to select for the high GSM bands and UMTS. The signals are separated an octave in frequency, making the diplexer a simple and inexpensive component.
Positioned on either side of the diplexer are switches that further determine which path is connected to the antenna. For GSM, either the transmitter or the receiver is engaged. For WCDMA, both Tx and Rx are on concurrently and must be kept distinct with a duplexer. Conceptually the same as a diplexer, a duplexer is more complex as it has the requirements of separating two signals in that are very close in frequency.
In FIG. 1, the handset can operate in three modes: GSM 900, GSM 1800, or WCDMA. In addition, the diplexer enables the handset to operate in GSM 900 while looking for service in WCDMA or operate in WCDMA while looking for service in GSM 900. Since there is no filtering between them, the handset cannot simultaneously operate in GSM 1800 and WCDMA.
The circuit shown in FIG. 2 supports GSM in four frequency bands. This allows the handset to operate in North America as well as the rest of the world. The UMTS spectrum was not allocated in North America, and so the handset can only operate in the GSM mode there.
A cellular handset supports simultaneous service for different frequency ranges, and different wireless standards. The handset includes an antenna connected to a bandpass filter and a first notch filter. The bandpass filter passes the receive band of the UMTS band. A diplexer connects to the output of the first notch filter and several antenna switches. In a first signal path, the antenna switch connects either the transmission or the reception for GSM 900. In a second signal path, the antenna switch connects to a second notch filter and a duplexer. The second filter passes the transmit band of GSM 1800. The duplexer passes the transmit band of UMTS and the receive band of GSM 1800.
Alternatively, the handset includes an antenna connected to a bandpass filter and a first notch filter. The bandpass filter passes the receive band of the UMTS band. A diplexer connects to the output of the notch filter and several antenna switches. In a first signal path, the antenna switch connects either the transmission of GSM 850/900, GSM 850 reception, or GSM 900 reception. In a second signal path, the antenna switch connects to a second notch filter, GSM 1900 reception, and a duplexer. The second notch filter passes the transmit bands of GSM 1800/1900. The duplexer passes the transmit band of UMTS and the receive band of GSM 1800.
Alternatively, the handset is as above, but the diplexer includes a notch filter.